It's 1 a.m. and 23-year-old web designer "Isabel" is riding the uptown No. 1 train. She's exhausted and ready to go to sleep, when a young man sitting across from her piques her attention. He has brown hair and green eyes, and is reading a Hebrew book. Sleepy and shy, she tries to think of a good opening line. She almost has a witty one ready, when the man jumps off at the 96th street stop.

Such random encounters and missed connections are everyday occurrences.

But while Isabel (not her real name) may never see the young man again, and the odds of locating him may be remote, all is not lost - thanks to "missed connections" Web sites, including those at the popular online bulletin board Craigslist.

Craigslist hosts online communities for many major cities including San Francisco, Boston, New York, Washington and Los Angeles. Aside from "help-wanted" ads and apartment listings common to most community sites, one of Craigslist's most popular sections is "missed connections."

On the N/W train Monday evening. You: Attractive, 30ish, brown hair, soft smile, standing and holding the pole. Me: Black coat and black bag. You saw me smile at you - I looked away. I wish I hadn't. Would love to talk to you - please write soon."

Included in the post is an anonymous e-mail address people can reply to. Posters often seem shy, or for some unfortunate reason fail to get (or keep) contact information.

HAPPY ENDINGS NOT GUARANTEED

Newspapers often run "missed connections" ads alongside personals. With the advent of online dating, looking for "the one" over the Internet has taken off. And people are now looking for "the one who got away" online, as well.

One of the things that has made these online sites a hit is volume. The "missed connections" section of Craigslist (for all cities) boasts of 6.5 million hits - with 14,000 postings in the month of January alone. After all, the probability of locating a mystery man or woman is increased when one posts on a popular site - and both posting and browsing are free.

And even when a missed connection is found, things don't always go as planned.

Scott Blake of San Francisco started habitually going through the "missed connections" site after reading about it in a paper. He initially just found it amusing.

"It helped me get a feel for the San Francisco dating jungle," says the divorced 47-year-old. He was a regular browser of the Craigslist bulletin board, when he found a posting about a man that fit his own description.

"It was about a guy who approached a woman in Target a couple of days before Christmas," says Blake. The posting was extremely specific: It listed the date, the store location, the woman's attire and included that the man was looking for vacuum bags.

"'Wait a minute,' I thought," says Blake. "I think it's about me."

Blake wrote the woman, described himself and they agreed it was a match. She, however, became so skittish about meeting him that the e-mail correspondence eventually stopped.

"The story doesn't end well," he says. "That pretty much sums up my exasperation with dating. Postings can work, but it's still up to the person initiating the search to carry through with the contact," he says.

And some myserious strangers probably should have stayed mysterious. One New York man says he posted an ad to find a man he saw on the subway, and was shocked when he got a reply from the right person.

"I never thought it would happen, and to me of all people," says the 25-year-old, who didn't want his name used. "It was kind of cool: We e-mailed several times, talked on our cell phones, and set a place and a date to meet."

The meeting itself, however, was ill-fated. After the man's date casually informed him that he was married with two children, the 25-year-old thanked him for replying to the ad and then walked out of the restaurant.

"Craig helps you find them," the man says, "but after that, he has no control."

COMMUNITY IN SEARCH OF LOVE

Craiglist's "missed connections" bulletin board has become more than just a venue to find lost and potential soulmates, it's now a real community. People regularly revisit the site to read postings, and the tone of the discussion, unlike, say, on the "casual encounters" thread, is distinctly romantic and hopeful.

"I started this section because I was always affected by those World War II movies where people wouldn't get together until years later, or would never find each other at all," says Newmark. "It appealed to the romantic side of me."

The same apparently holds true for its avid readers.

"I used to read it to laugh at how absurd some people were," Isabel says. "In a city of hundreds, some of these ads never make sense. Then it became an obsessive 'I-wonder-if-I-know-anyone-posting.' And what if I do post, will whoever I post about ever find me?"

Stephanie, a 27-year-old copywriter who didn't want her last name used, reads the postings as a form of non-fiction entertainment, akin to reality TV.

"Sometimes, you'll find people who've had the same heartaches you've had," she says.

If the threads on the site are accurate, people have actually gotten together through "missed connection" postings, even if the person they end up with is not the one they originally wrote about.

Posters have also expanded the definition of "missed connections." Some write about friends they can't confess their feelings to, and crushes they can locate but cannot approach. Jilted lovers even use the bulletin board to send their "Dear John" letters out into the universe.

Threads have also included discussions on dating in the city, the best Valentine's Day presents, and the age-old debate on the difference between men and women.

And even if not everyone finds those they "missed," the site is another example of the way the Internet is now intermingled with our "offline" lives, combining real-life interaction (often ill-fated, as in this case) with an online community.

So, does Isabel ever find her green-eyed boy?

No. But that hasn't kept her from continuing to browse the site. It's become a virtual support group for her as she searches.

"I'm now more aware of the exchange of looks and smiles when I'm in public places, and I can't help but feel like it's 'missed connection'-mode all the time," Isabel says.

In the meantime, she's learning to be a little braver and a little more forward so that next time, she won't miss that connection.